Thermal head

ABSTRACT

A thermal head for a thermal-transfer or heat-sensitive printer has a silicon substrate having a projection. A porous silicon layer having a small thermal conductivity is formed on the silicon substrate so as to cover at least the projection on the silicon substrate. A heat-generating resistor layer is formed on the porous silicon oxide layer in a region above the projection of the silicon substrate. A conductor layer is formed on the heat-generating layer so as to expose the portion of the heat-generating resistor layer which is right above the projection of the silicon substrate and which serves as a heat-generating portion. The heat-generating portion and the conductor layer are covered by a protective layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal head for use in thermaltransfer type printers or heat-sensitive type printers and moreparticularly, to a thermal head of the type in which a heat-generatingportion is provided on a projecting portion of a substrate.

2. Description of the Related Art

FIG. 18 shows a known thermal printer head. This thermal printer headhas an aluminum substrate 1 which is locally covered by aheat-accumulation layer 2 of glass glaze. A heat-generating resistorlayer 3 is formed on the heat-accumulation layer 2 and the substrate 1.A conductor layer 4 for supplying electrical current to theheat-generating resistor layer 3 is formed on the layer 3. Aheat-generating portion 6, composed of the resistor layer 3 exposedthrough the conductor layer 4, is formed in a dot-like form. Aprotective layer 5 is formed on the dot-like heat generating portion 6to protect the latter from oxidation and wear.

This thermal head is used in contact with a recording medium (not shown)such as an ink ribbon or a heat-sensitive paper. When an electricalcurrent is supplied to the heat-generating portion 6 of the thermalhead, heat is generated to transfer an ink in an ink ribbon to therecording medium or enables coloring component in the recording mediumto develop a color, whereby information is recorded on the recordingmedium.

The thermal response characteristic is improved to enable high-speedprinting when the glass glaze heat-accumulation layer is thinned asshown in FIG. 19. In such a case, however, the pressure at which thedot-like heat-generating portion 6 is pressed against the ink ribbon isreduced because the amount of projection or height of theheat-generating portion 6 is reduced correspondingly. This causesvarious inconveniences such as impairment of printing quality andefficiency, as well as increase in the electrical power consumption.Printing quality and efficiency would be improved when the projectionheight of the heat-generating portion 6 is increased by an increase inthe glass heat-accumulation layer 2 as shown in FIG. 20, because in sucha case the heat-generating portion 6 can be pressed onto the ink ribbonat higher pressure. In such a case, however, the thermal capacity of thethermal head is increased due to increase in the thickness of the glassglaze heat-accumulation layer 2, with the result that the thermalresponse characteristic of the thermal head is impaired to, and thusinhibits high-speed printing.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a thermalhead which offers an improvement in printing efficiency, i. e. areduction in the electrical power consumption, without being accompaniedby degradation of the printing quality.

To this end, according to one aspect of the present invention, there isprovided a thermal head comprising: a silicon substrate having aprojection; a porous silicon oxide layer formed at least on theprojection of the silicon substrate; a heat-generating resistor layerformed on the porous silicon oxide layer in the region above theprojection of the silicon substrate; a conductor layer formed on theheat-generating resistor layer so as to expose only a portion of theheat-generating resistor layer which is positioned above the projectionand which serves as a heat-generating portion; and a protective layerformed to cover the heat-generating portion and the conductor layer.

According to a second aspect of the invention, there is provided athermal head comprising: a silicon substrate having a projection; aporous silicon oxide layer formed to cover only the crest of theprojection of the silicon substrate; a heat-generating resistor layerformed on the porous silicon oxide layer in the region above theprojection of the silicon substrate; a conductor layer formed on theheat-generating resistor layer so as to expose only a portion of theheat-generating resistor layer which is positioned above the projectionand which serves as a heat-generating portion; and a protective layerformed to cover the heat-generating portion and the conductor layer.

The term "crest of the projection" is used in this specification to meana region encircled by a circle which is centered at the center of theprojection and which has a diameter about 5 to 10 μm greater than thedistance between the portions of the conductor layer opposing each otheracross the heat-generating portion and having a depth of 10 to 40 μm thethicknesswise direction of the silicon substrate.

The thermal head in accordance with the first aspect of the presentinvention provides superior printing quality because the heat-generatingportion, which is provided on the crest of projection formed on asilicon substrate, can be pressed onto an ink ribbon or a recordingmedium such as a heat-sensitive paper at a sufficiently high pressure.In addition, the porous silicon oxide layer having a small thermalconductivity, provided at least on the projection of the substrate,reduces the rate of transfer and conduction of heat from theheat-generating resistor layer to the silicon substrate, thus improvingprinting efficiency.

The effect to reduce the heat transfer and conduction is not appreciablewhen the thickness of the porous silicon oxide layer is below 10 μ,whereas any porous silicon oxide layer thickness exceeding 100 μm causesthe heat transferred to the porous silicon oxide layer to be accumulatedin this layer, with the result that the thermal response characteristicis impaired to inhibit high-speed printing.

The thermal head in accordance with the second aspect of the inventionalso offers improvement in the printing quality because theheat-generating portions, which are provided on the crest of projectionformed on a silicon substrate, can be pressed onto an ink ribbon or arecording medium such as a heat-sensitive paper at a sufficiently highpressure, as in the case of the thermal head of the first aspect.Furthermore, the porous silicon oxide layer having small thermalconductivity is provided only on the crest of the projection of thesilicon substrate so that the area of boundary between theheat-generating portion and other portions of the substrate per unitarea of the porous silicon oxide layer is increased. In addition, thesubstrate is made of a silicon wafer which exhibits greater thermalconductivity than alumina which has been used conventionally. Therefore,when the thermal head operates, the transfer of heat from theheat-generating portion to the substrate is reduced, as compared withthe case where the porous silicon oxide layer is not provided, whereas,when the heating is terminated, the time required for the temperature ofthe heat-generating portion to drop is shortened as compared, with thecase where the porous silicon oxide layer is formed over a wide area,thus enabling highspeed printing.

The above and other objects, features and advantages of the presentinvention will become more clear from the following description of thepreferred embodiments when the same is read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an embodiment of a thermal head inaccordance with the present invention;

FIGS. 2 to 16 are sectional views of the the embodiment of the thermalhead shown in FIG. 1 in different steps of a process for producing thethermal head;

FIG. 17 is a sectional view of a second embodiment of the thermal headin accordance with the present invention; and

FIGS. 18 to 20 are sectional views of conventional thermal heads withheat-accumulation layers formed of glass glaze.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a first embodiment of the thermal head inaccordance with the present invention has a substrate 7 which is made ofsilicon. The substrate 7 has a protrusion 20. The protrusion 20 has aprojection 8 which projects to form a crest of the protrusion with asteep slope and a peripheral skirt portion 10 of a gentle slope orgradient surrounding the projection 8. A porous silicon oxide layer 16of 10 to 100 μm thick is formed on the substrate 7 so as to cover theprotrusion including the projection 8 and the skirt portion 10. An SiO₂film 14 is formed on the regions of the silicon substrate where theporous silicon oxide layer 16 is not formed. The porous silicon oxidelayer 16 and the SiO₂ film 14 are covered by a heat-generating resistorlayer 3 made of, for example, Ta₂ N, Ta-Cr-N, Ta-SiO₂ or the like andhaving a thickness ranging between 0.05 and 0.3 μm. On theheat-generating resistor layer 3 is formed a conductor layer 4 so as tolocally expose the heat-generating resistor layer 3 in theheat-generating portion 6 above the projection 8, the conductor layer 4being made of, for example, Al, Ni-Cr/Au or the like and having athickness of 1 to 2 μm. A protective layer 5 of 5 to 7 μm and made of,for example, SiO₂ /Ta₂ O₅, thialone or the like, is formed to cover theconductor layer 4 and the heat-generating portions 6 exposed through thelatter.

In this embodiment, the heat-generating portion 6, which is formed abovethe projection 8 of the protrusion 20 of the silicon substrate 7, can bepressed onto the ink ribbon or the recording medium such asheat-sensitive paper, thus ensuring high printing efficiency andquality.

Furthermore, the porous silicon oxide layer 16 of 10 to 100 μm thick,formed on the substrate 7 to cover the projection 8 and the peripheralskirt portion 10 of the protrusion 20, serves to suitably reduce thetransfer of heat from the heat-generating resistor layer 3 to thesilicon substrate 7.

This thermal head, therefore, can operate with a reduced electricalpower consumption, contributing also to the improvement in the printingefficiency.

PRODUCTION EXAMPLE

An example of the process for producing the thermal head of the firstembodiment will be described with specific reference to FIGS. 2 to 17.The process is composed mainly of three major steps: namely, a step forpreparing a silicon substrate having, a projection, a step for forming aporous silicon oxide layer on the substrate, and a step for fabricatingthe thermal heat using the substrate having the porous silicon oxidelayer.

The first step for forming the silicon substrate with projections wascarried out as follows.

(1) A silicon substrate 7 as shown in FIG. 2 was prepared. The siliconsubstrate 7 was a P-type substrate having a resistivity of 0.01 Ω·cm.

(2) A masking liquid (CBR-M901 produced by Nippon Gosei Gomu) wasapplied to the silicon substrate 7 by spin coating. The siliconsubstrate 7 was then exposed though the masking liquid and thensubjected to a development, whereby a mask 18 was formed as shown inFIG. 3. The width of the mask was 500 μm.

(3) Subsequently, etching was conducted for 3 minutes by using an acidicmixture, composed of 79 wt % of fluoric acid, 3 wt % of nitric acid and18 wt % of acetic acid. As a result, the silicon substrate 7 was etchedto a depth of about 30 μm. As shown in FIG. 4, the etching was effectedsuch that portions of the silicon substrate beneath each mask 18 waspartly removed.

(4) The mask 18 was then separated by using a mixture solution ofsulfuric acid and aqueous hydrogen peroxide, whereby a protrusion 9having edges 19 were left on the silicon substrate 7.

(5) Subsequently, the silicon substrate 7 was immersed for 6 minutes inan acidic mixture containing 10 wt % of fluoric acid, 80 wt % of nitricacid and 10 wt% of acetic acid so that edges 19 were removed to leavethe protrusion 9 in a form as shown in FIG. 6.

(6) Then, a masking liquid (CBR-M901 produced by Nippon Gosei Gomu) wasapplied to the silicon substrate 7 by spin coating method and thesubstrate 7 was subjected to exposure and development, whereby a mask 18having a width of 100 μm was formed on the center of the siliconsubstrate 7.

(7) Then, etching was conducted for 1 minute by using the same acidicmixture as that used in the previous step (3), whereby the substrate 7as etched at its region along the periphery of the mask 18 to a depth ofabout 10 μm.

(8) Then, the mask 18 was removed by using a mixture liquid of sulfuricacid and aqueous hydrogen peroxide as in the previous step (4), wherebya projection 8 of about 10 μm tall was left on the center of theprotrusion on the silicon substrate 7.

(9) The silicon substrate 7 was then etched by the same acidic mixtureas that used in the previous step (5) so as to remove edge 21 of theprojection 8, whereby a protrusion 10 including a central projection 8and a peripheral skirt portion 10 was left on the silicon substrate asshown in FIG. 10.

The second step for forming the porous silicon oxide layer was conductedin the following manner.

(1) As shown in FIG. 11, a mask 11 of a photo-resist (CBR-M901 producedby Nippon Gosei Gomu) was formed on the silicon substrate 7 of FIG. 10.The mask 11 was partly removed to expose the surface of the siliconsubstrate 7 by photolithographic etching at the portion thereof wherethe porous silicon oxide layer is to be formed.

(2) Meanwhile, 20 wt % aqueous solution of hydrofluoric acid was chargedin an electrolytic cell having a platinum plate serving as a cathode,and the silicon substrate 7 was immersed in the solution so as to facethe platinum plate and so as to serve as an oxide. Anodic chemicalconversion treatment was then effected with a D.C. current. The currentdensity and the treating time were respectively 50 mA/cm² and 20minutes.

The photo-resist mask 11 formed on the silicon substrate 7 couldfunction as a mask without fail, because it was not corroded by hydrogenfluoric acid. As a consequence, a porous silicon layer 15 having athickness of 40 μm and a porosity of 80% was formed only on the portionof the silicon substrate 7 devoid of the mask 11, i.e., only on theregion 12 where the porous silicon oxide layer is to be formed. Thethickness of the porous silicon layer 15 could be freely controlled byvarying the time of the anodic chemical conversion.

(3) After a sufficient rinsing, the silicon substrate 7 was subjected toa step for removing the mask 11 by a mixture liquid of fluoric acid andaqueous hydrogen peroxide.

(4) The silicon substrate was then sufficiently rinsed and left in theatmospheric air at 850° to 1000° C. for heat oxidation, whereby theporous silicon layer 15 was oxidized. As a result, the porous siliconlayer 15 was changed into a porous siliconoxide layer 16 covering theprotrusion 20 and a surrounding region as shown in FIG. 14. The surfaceof the silicon substrate 7 also was oxidized during the above-mentionedheat-oxidation, whereby an SiO₂ film 14 of 0.05 to 0.1 μm, thick wasformed around the porous silicon oxide layer 16.

The silicon substrate thus prepared may be coated, as necessitated, witha film of non-porous silicon oxide or a non-porous insulating film suchas thialon formed by sputtering.

The third step for fabricating the thermal head using this substrate wasconducted in the following manner.

(1) A heat-generating resistor layer 3 as shown in FIG. 15 was formed onthe silicon substrate 7 having the porous silicon oxide layer 16 bysputtering with Ta₂ N, Ta-Cr-N or Ta-SiO₂. The thickness of thisheat-generating resistor layer 3 was 0.05 to 0.3 μm.

(2) Subsequently, a conductor layer 4 of 1 to 2 μm thick (2)Subsequently, was formed on the heat-generating resistor layer 3 byevaporation form A or Ni-Cr/Au. and the portion of the conductor layer 4above the projection 8 was removed by photolithographic etching, thusforming a heat-generating portion 6, whereby the heat-generatingresistor layer 3 and the conductor layer 4 were connected together atboth sides of the porous silicon oxide layer 16.

(3) Then, a protective layer of 5 to 7 μm thick was formed by sputteringfrom SiO₂ /Ta₂ O₅ or thialon, whereby a thermal head as shown in FIG. 1was completed.

A description will now be given of a second embodiment of the invention.

FIG. 17 shows a second embodiment of the thermal head of the presentinvention. The second embodiment is basically the same as the firstembodiment except that the porous silicon oxide layer of a specificthickness, preferably 10 to 40 μm, is formed only on the crest of theprojection 8.

In this embodiment,therefore, the heat-generating portion 6 is providedabove the projection 8 of the protrusion on the silicon substrate 7, sothat the heat-generating portion 6 can be pressed at a sufficiently highpressure to an ink ribbon or a recording medium such as a heat-sensitivepaper, thus ensuring high printing quality as in the case of the firstembodiment.

In the second embodiment, the porous silicon oxide layer having a smallthermal conductivity is provided to have a thickness of 10 to 40 μm onlyon the crest of the projection 8 so as to minimize the area of boundarybetween the porous silicon oxide layer 16 and the substrate 7 per unitarea of the porous silicon oxide layer. In addition, a silicon waferwhich has greater thermal conductivity than the conventionally usedalumina is used as the material of the substrate 7. Therefore, duringprinting operation, transfer and conduction of heat from theheat-generating portion to the substrate 7 is reduced as compared withthe case where the porous silicon oxide layer 16 is not provided, thusoffering a higher printing efficiency. On the other hand, when thegeneration of heat is caused, the temperature of the heat-generatingportion 6 can be lowered in a shorter time than in the case where theporous silicon oxide layer is formed over a wider area, i.e., ascompared with the thermal head of the first embodiment, therebyattaining a higher printing speed.

The term "only on the crest of the projection 8" is used to mean aregion encircled by a circle which is centered at the center of theheat-generating portion 6 and which has a diameter about 5 to 10 μmgreater than the distance between the portions of the conductor layer 4diametrically opposing across the heat-generating portion 6.

The thermal head of this embodiment could be fabricated substantially bythe same process as that for the thermal head of the first embodiment,except that the mask 11 used in the sub-step (1) in the second step ofthe process for the first embodiment was sized and shaped to cover onlythe crest of the projection 8.

As will be understood from the foregoing description, the presentinvention offers various advantages as follows.

First of all, it is stressed again that the printing quality can beremarkably improved by virtue of the fact that the heat-generatingportion, which is formed above the projection on the silicon substrate,can be pressed onto an ink ribbon or a recording medium such as aheat-sensitive paper with sufficiently high contact pressure.

In particular, in the first embodiment of the invention, conduction andtransfer of heat from the heat-generating resistor layer to the siliconsubstrate can be suppressed by the presence of the porous silicon oxidelayer of 10 to 40 μm thick having a small thermal conductivity andcovering the projection and the peripheral region surrounding theprojection on the silicon substrate. As a consequence, electrical powerconsumption is reduced, which offers an additional advantage ofimprovement in printing efficiency in addition to the above describedadvantage of improved printing quality.

In the second embodiment, the porous silicon oxide layer 16 having smallthermal conductivity is provided only on the crest of the projection inthe silicon substrate so as to minimize the area of boundary between theporous silicon oxide layer and the heat-generating area. At the sametime, a silicon wafer having a greater thermal conductivity thanconventionally used alumina is employed as the material of thesubstrate. Therefore, during the printing, the transfer and conductionof heat from the heat-generating portion to the substrate is reduced ascompared with the case where the porous silicon oxide layer is notprovided, whereby the printing efficiency is increased. On the otherhand, when the generation of heat is terminated, the temperature of theheat-generating portion can be lowered in a shorter time than in thecase where the porous silicon oxide layer is formed over a wider area,i.e., as compared with the of first embodiment of the thermal head, thusattaining a higher printing speed.

Thus, the present invention provides a thermal head which is superior inprinting quality, printing efficiency and printing speed as comparedwith known thermal heads.

What is claimed is:
 1. A thermal head comprising:a silicon substratehaving a projection defining a peripheral portion and acentrally-located crest; a porous silicon oxide layer formed on saidprojection; a heat-generating resistor layer formed on said poroussilicon oxide layer; a conductor layer formed on said heat-generatingresistor layer, said conductor layer defining an opening disposedadjacent said crest; and a protective layer formed to cover saidheat-generating
 2. A thermal head according to claim 1, wherein saidporous silicon oxide layer is formed only on the crest.
 3. A thermalhead comprising:a silicon substrate having a projection comprising aperipheral region having a first surface defining a first slope, a crestregion centrally located within said peripheral region having a secondsurface defining a second slope and a peak region, said second slopebeing greater than said first slope, and a porous silicon oxide regionformed in said central crest region; a heat-generating resistor layerformed on said second surface; and a conductor layer formed on saidheat-generating resistor layer over said second slope and defining anopening such that said peak region is exposed.
 4. A thermal headaccording to claim 3 wherein said porous silicon oxide region is formedin said peripheral region and said crest of said projection.
 5. Athermal head according to claim 4 wherein said porous silicon oxideregion is 10 to 100 μm thick.
 6. A thermal head according to claim 5wherein said porous silicon oxide region is approximately 40 μm thick.7. A thermal head according to claim 3 wherein said heat-generatingresistor layer formed on said second surface and said first surface. 8.A thermal head according to claim 7 wherein said conductor layer isformed on said heat-generating resistor layer adjacent said second slopeand said peripheral region.
 9. A thermal head according to claim 3further comprising a protective layer formed on said projection.